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The  photograph shows an early Potter-Bucky Diaphragm, some timed referred to as an Anti-scatter Grid.. The first models were built up from alternate strips of lead and wood. The strips are built up on a radius which would have at its centre the x-ray tube anode. 
When radiation encounters some form of matter, some of the radiation is scattered in all directions. When this scatter radiation reaches the film, it does not contribute to the final image, but simply produces an overall fog level. The amount of scatter produced increases as the kV is increased, and is also proportional to the volume irradiated.
The Potter-Bucky diaphragm removes most of the scatter while allowing most of the primary radiation through. The lead slats would be expected to cast a shadow on the image, but this is removed by moving the grid during the exposure. The rails on which the grid moves can be seen in the top
right of the photograph.
The modern version of the Potter -Bucky diaphragm is flat instead of curved, but employs the same principle.